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The multipath effect is caused by the reflection of the GPS signal on nearby surfaces or objects such as tall buildings. It is known that large bodies of waters are the most reflective and therefore detrimental to the GPS accuracy. This effect occurs when the signal, produced by the satellite, reaches the receiver by more than one path due to an obstruction. The reflected signal then takes a longer amount of time to reach the receiver than the direct signal. This allots to an inaccuracy of anywhere between 0-20 meters but is highly within the 0-1 meter range.
 
The multipath effect is caused by the reflection of the GPS signal on nearby surfaces or objects such as tall buildings. It is known that large bodies of waters are the most reflective and therefore detrimental to the GPS accuracy. This effect occurs when the signal, produced by the satellite, reaches the receiver by more than one path due to an obstruction. The reflected signal then takes a longer amount of time to reach the receiver than the direct signal. This allots to an inaccuracy of anywhere between 0-20 meters but is highly within the 0-1 meter range.
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[[Image:MultiPath.jpg]]
  
 
===Receiver clock errors ===
 
===Receiver clock errors ===

Revision as of 12:46, 3 March 2008

Contents

Executive Summary

This section is much like an abstract and summarizes the entire report

Introduction

This section should include an introduction of the product and a brief description of group members (i.e. who was responsible for which sections or tasks)

Introduction to GPS - John Bonck

How GPS Works - John Bonck

WAAS - John Bonck

Sources of Error - Lindsay Woods

Improving Accuracy & DGPS - Lindsay Woods

Uses of GPS - Tim Whittendale

Brief History of Navigation - AJ Leonard

Calculating Positions with GPS - Alison Lupariello

GPS Software - AJ Leonard

DeLorme Earthmate GPS PN-20 Information/Disassembly/Assembly - Jonathan Istranyi


Global Positioning System or GPS is a satellite-navigation system that consists of a network of 24 satellites placed into orbit by the U.S. Department of Defense. Originally GPS was solely intended for military use, costing the U.S. billions of dollars to build and maintain. Finally in the 1980's the government made the system available for civilian use. GPS was used by hikers to navigate through the wilderness and by boaters and fishermen to find their whereabouts in the open sea and surveyors use GPS to position themselves on a particular site within one centimeter of their intended destination. The most popular GPS application and the most recent is the navigation system within a motor vehicle.

A Brief History of Navigation

How GPS Works

For the GPS (in this case Earthmate PN-20) to know where it is located, the GPS needs to know where the satellites location and distance as compared to the GPS. The GPS receives two different types of data the almanac and ephemeris. The almanac data contains approximate positions (locations) of the satellites, which is continuously transmitted and stored in the memory of the GPS receiver so it knows where the orbits of the satellites and where each satellite is suppose to be. If a satellite travels slightly out of orbit, ground monitoring stations are always keeping track of the satellite orbits, altitude, location and speed. The ground monitoring stations are always sending data to a master control station, which in turn sends corrected data up to the satellites, this is corrected data is known as ephemeris. Once the GPS receiver knows the location of a satellite, the receiver must determine the distance between the receiver and the satellite so that it can determine its position on earth. The velocity of the radio wave is 186,000 miles per second (speed of light) and the time it takes for the signal to get to the receiver determines the distance between the receiver and the satellite, thus giving the GPS receiver a location.

There are many sources of error associated with GPS. Signals can be slowed as it passes through the atmosphere, the system uses a built-in model that calculates an average, but no exact amount of delay can be determined. Signal multi-path errors originate from signals reflecting off of tall buildings or large rock sources before it reaches the receiver. These same tall buildings can also block signals from satellites, the smaller amount of satellites the less accurate the GPS becomes.

W.A.A.S.

Wide Area Augmentation System or WAAS was first created by the Federal Aviation Administration (FAA) to provide exceptional positioning information when pilots are found in deteriorating weather conditions or when visual navigation becomes impossible. A network of 25 ground reference stations or Wide Area Reference Stations (WRS) that cover the entire U.S. and some of Canada and Mexico. Each station is in a precisely surveyed location where they compare GPS distance measurements to known values. Each station is connected to the WAAS Master Station (WMS) where GPS information is collected and forwarded to through a terrestrial communications network. At the WMS, the WAAS augmentation messages are generated these messages contain information that allows GPS receivers to remove errors in the GPS signal. The errors removed from the signal increase the accuracy of the measured location. In aircrafts WAAS receivers typically have accuracies of 3 to 5 meters horizontally and 3 to 7 meters in altitude.

WAAS is now being used by regular GPS receivers for a more precise location. WAAS also provides indications to GPS?WAAS receivers of where the GPS system is unusable due to system errors or other effects. Also the WAAS system was designed to notify users within six seconds of any issuance of hazardously misleading information that would cause an error in the GPS position estimate.

Sources of Error

Selective Availability

Atmospheric Distortion

Relativistic effects

Noise, bias and blunder

Signal multi-path

The multipath effect is caused by the reflection of the GPS signal on nearby surfaces or objects such as tall buildings. It is known that large bodies of waters are the most reflective and therefore detrimental to the GPS accuracy. This effect occurs when the signal, produced by the satellite, reaches the receiver by more than one path due to an obstruction. The reflected signal then takes a longer amount of time to reach the receiver than the direct signal. This allots to an inaccuracy of anywhere between 0-20 meters but is highly within the 0-1 meter range.

MultiPath.jpg

Receiver clock errors

Ephemeris Errors

Improving Accuracy & DGPS

Uses of GPS

Global Positioning System, GPS, has a wide variety of uses for the military as well as everyday applications for the private sector.

Private Sector Uses

The most common use of GPS in the private sector is for navigation. Navigation is becoming a standard feature in most automobiles. The navigation units in these automobiles do more than take you from point A to point B, they also list numerous points of interest, helping people unfamiliar with an area find these locations. This GPS navigation is not only limited to automobiles, it is used in boats, as well as planes. On commercial, as well as recreational, boats, GPS is used to navigate around the waters. It provides a much more accurate navigation assistance than the manual navigation tools from years past. Airplanes, commercial and private, use GPS technologies as well. Air traffic control towers use the GPS technology to track every plane that is in the air. The information that is gathered can help prevent disasters, such as in air collisions. The pilots of the plane use GPS to help them stay on route, preventing such disasters as well.

Hand-held GPS units are being used for a wide variety of recreational activities. Snowmobiling, hiking, biking, hunting, etc., are some of the recreational activities that use GPS devices to help navigate through the woods, mountains, and other areas. It allows these people to venture off the trails without fear of getting lost.

The majority of cellular phones are now being equipped with GPS capabilities. These phones are able to give turn by turn directions. These GPS capabilities also allow that phones location to be pin-pointed. This can be done on a personal computer, but more importantly, it is used by 911 services to find out the location where emergency calls are placed.

The use of GPS is starting to be used in the construction field. GPS devices are being installed on earth moving equipment in order to make grading job sites easier. The device that is mounted on the machine works with earthwork design programs such as Agtek. The site file, with the correct grades, is loaded into the on board computer, and the program shows the operator what level to have the blade, in order to achieve the correct grades. This technology is eliminating the need for a surveyor to stake the site before grading can begin. GPS surveying equipment is also allowing great advances in the study of earthquakes.

Surveyors are using GPS technology to get basic information about a site that they are working. This information used to be gathered by using optical surveying devices as well as other measuring devices. A surveyor would start with a known point and then use these devices to find out information about other points on the site. In order to complete the gathering of information on a site, the surveyor and the crew would spend numerous days on the site, and only be able to gather the information during the daylight hours. With the use of GPS, the time needed to gather information in the site is reduced drastically. Since it is uses satellites to obtain the needed information, the surveyor could work into the night. The GPS unit works by using satellites to record the different signals that are received when the device is taken to the desired locations. All of the gathered information can be logged into the computer, and the necessary maps could be made.

Military Uses

Like the private sector, the military uses GPS for navigation purposes. It helps the soldiers coordinate movements for supplies and missions. It also gives them the ability to move at night or in an area that they don't have much information on. When gathering information on an area, reconnaissance missions are performed, these missions, coupled with GPS technologies allow maps to be created of the area to aid in war.

Soldiers and pilots are being equipped with GPS receivers, so that in the event that they are lost, or an aircraft goes down, it is easier for their fellow soldiers to find and rescue them.

GPS is used to track and identify possible enemy targets. These GPS units are able to track targets on the ground and in the air. Once the target is confirmed to be an enemy, it can be engaged by the many GPS guided weapons. Missiles, precision guided munitions, etc. Having the exact coordinates of the enemy, and the use of the GPS, minimizes civilian casualties.


Calculating Positions with GPS

(Alison Lupariello)

Satellites in 6 Orbital Planes Satellites in 6 Orbital Planes

There are three main components of the GPS system and these include: the satellites that transmit the position information, the ground stations that control these satellites, and the individual GPS receivers that collect the data from the satellites and calculate the exact locations.


curves of overlapping pseudoranges Curves of Overlapping Pseudoranges

DeLorme Earthmate GPS PN-20

GPS Software

General Information

The unit being disassembled below is the DeLorme Earthmate GPS PN-20. It was received in new condition. The entire contents of the package are shown below:

The DeLorme Earthmate GPS PN-20 The entire contents of the DeLorme Earthmate GPS PN-20 package


The package includes: GPS unit, installation CD, topographic map CDs, operations/maintenance manuals, carrying bag, memory card, vehicle charger/adaptor, A/C wall adaptor, cable connectors (USB and eight-pin) as well as LCD covers for the unit's screen.


This GPS unit is compatible with computers, cars, and boats. It can also be used as a handheld device with map information stored on a SD memory card (1GB SD card included with package) that is inserted behind the two AA batteries that are used to power the device (see diagrams below for more information. The device is very sturdy yet compact, easily concealed yet able to withstand the elements for wherever you go.


Disassembly Procedure

A basic Philips Head computer screwdriver. The only tool required for disassembly/reassembly.


The disassembly of the DeLorme Earthmate GPS PN-20 is a fairly simple procedure. It requires the use of only one 1/16" computer screwdriver.

The front of the DeLorme Earthmate GPS PN-20. LCD display along with button user interface. The back of the GPS Unit. Eight-pin connector at the top with the battery plate beneath it.


1. The first step to disassemble the unit is to flip it over to the back side and remove the two screws holding the battery plate in place. This can be done by hand as the two screws have handles for ease of untwisting the screws.

Back of the GPS unit with the battery plate and accompanying screws removed.


2. The second step to disassemble the unit is to unscrew the remained eight screws holding the bottom half of the GPS unit to the top half. For this step a screwdriver is required. The top two screws (near the lanyard loop) are the smallest screws. The remaining six screws (all of equal length and size) are fastened into place with rubber washers most likely to maintain the unit's waterproof case, two screws of the remaining six are on the sides of the eight-pin connector and the remaining four screws are removed from the wells behind the now removed battery plate. *Note* The four screws behind the battery plate, while easily unfastened from the top half of the GPS unit are difficult to remove from their respective wells and thus are not shown in the corresponding diagram.

The GPS unit split into two halves with the screws removed.


3. The third step to disassemble the unit is to remove the upper and lower halves of the GPS unit now that the screws that have held them in place have been removed. Once opened it should resemble the corresponding image shown above on this page. The innards of the Earthmate GPS PN-20 consist of a slot to insert a SD card for topography data and other various maps, several wires connecting to both the battery power supply and the eight-pin connector. The motherboard itself contains various chips and circuits for processing data, displaying the LCD screen, and operating the button user interface on the front of the GPS unit.


After Disassembly

Part Table:

Earthmate GPS PN-20 Part table with numbered pieces.
Table of DeLorme Earthmate GPS PN-20 Components
Corresponding Part Number Part Name Number of Specified Parts Part Description
Label 1 GPS unit casing One Contains motherboard, LCD screen, power supply wires, SD card slot and other essentials for user operation
Label 2 Screws with rubber washers (half inch in length) Six Screws fastening the two halves of the GPS unit together. Two are placed on the sides of the eight-pin connector and the other four are placed underneath the battery plate.
Label 3 Screws (eighth inch in length) Two Screws fastening the two halves of the GPS unit together. These screws are placed near the top of the unit (near the lanyard loop)
Label 4 Screws with rotating handles (half inch in length) Two Screws that fasten the battery plate in place. Handles are attached to the screws so that a tool is not required to remove them.
Label 5 Battery plate One Plate that covers the battery power supply, also allows access to SD memory card slot

Assembly

Assembly of the DeLorme Earthmate GPS PN-20 follows the steps of disassembly except in reverse. The same tool, a simple Philip's head screwdriver, is all that is required. If the disassembly instructions were followed properly there should be difficulty reassembling the GPS unit.


After Assembly

After reassembling the GPS unit, insert two charged AA batteries and press the power symbol button on the front of the device. If the LCD screen powers up with access to all its functions, the device is ready for operation.


References

Langley, R. (2008). How does GPS Work? Retrieved February 25, 2008, from Geodesy and Geomatics Engineering Website: http://gge.unb.ca/Resources/HowDoesGPSWork.html -- Alison Lupariello

AN02 Network Assistance. Retrieved February 25, 2008, from Navsync GPS Technologies Website: http://www.navsync.com/notes2.html -- Alison Lupariello


APA Style You must use this format (It's easier than MLA, so don't worry).

Guide to Writing Wiki Code

The beauty about Wiki is that if you don't know the code, you can steal it from someone's page that does. Feel free to click the "edit" links or tabs to view the code for sections or the pages respectively. Be weary about wrecking havoc on another's page. Each page can be rollbacked to a previous verison and your username is linked to all changes. Although you might think it's cool to go through and insert "MIKE RULES" throughout the page, I'm sure Dr. Lewis would not be pleased.

Here's a few tips on writing with Wiki:

This is a bracket "[" "]"


This is a brace "{" "}"


To create a new page/link within Wiki:

  • Double brackets, page name, double brackets
  • Typing in a new page name will automatically create a page, which when clicked, you can then edit.
  • Whatever name you first type in is the name of the page. You can't change page names, only create new pages. Think before you create a new page.
  • Don't worry about slashes or anything, all pages are located in the same directory. If I wanted to create a page called "MAE 277 Template" the code would be ''MAE 277 Template'' Note: Brackets are italicized to prevent creating a new page.


Your table of contents is created automatically.

  • 1,2,3,4 are level 2 sections
  • 1.1, 1.2, 1.3 are level 3 headers


To create headers:

  • Section titles are wrapped with two equal signs ==My favorite header==
  • Bold headers within a section are wrapped with three equal signs ===My not-so-favorite header===


Asterisks indicate bullets. Be sure to put each asterisk on a new line.

  • Here's one
  • Here's two *Here's three, but its not on the next line


Bold text:

  • Start line with "b" in "<>". Be sure to end the line with "/b" in "<>" if you don't want the whole paragraph to be bold.
  • Surround text to be bolded with three " ' " marks on either side. Or highlight the text and click the "B" button on the toolbar.


Italics:

  • "i" in "<>". Don't forget to end with "/i" in "<>"
  • Highlight the text and click the "I" button in the toolbar (It will put four " ' " on either side).


This is a broken link media file caption


Media tags are indicated by "Media:", images by "Image:" Broken links in red. Case is not important. Use the toolbar to get examples if you're not sure.


Spacing is werid in wiki. Single return does nothing.

Double return (blank line), breaks the line.


Triple return (two blank lines), puts an extra blank line between lines of text.


"br" in "<>" will break lines. They can also be used to separate section headers.



Finally, use the "Show Preview" button on the bottom of the page to see how it looks before saving. It will allow you to catch and edit your errors without having to edit the page again. Just don't forget to save it when you're really done.

This is an example table

See help page for more information on the syntax.

This is the table title
This is Column Header 1 This is Column Header 2 This is Column Header 3
This starts Row 1 Width values (pixels) in header are used to designate the width of the column for the entire table. Text will wrap but it helps to control the layout. Height of the row is determined by the row's largest content A return and single vertical lines separate columns in rows. A double vertical line is necessary if you don't break up the text for cells.
This starts Row 2 "br" in brackets
break lines. Wiki sometimes ignores blank lines.
Some html tags can be used, but not many. Notice the align equals center tag at the beginning of the row. It centers the text in the first two columns, but doesn't work for the third column. I don't know why. Adding the tag again to the beginning of the cell in question will center the text.
This starts Row 3 Image tags are in this format:


Double brackets "[["
Image name
| = Vertical Line

The following order is not important, as long as each is separated by a vertical line:

  • Horizontal position (left, center, right)
  • Thumb (to create clickable thumbnail that links to fullsize image), don't include to make a fullsize
  • Size denoted in pixels (if desired)
  • You can add a caption if there is a thumbnail


Then close with double brackets "]]"

Broken links show up in red.



Here's where you can view any uploaded files

This is thumbnail
Camera disassembly 4.jpg


This is a resized image, not a thumbnail, but notice you can still click on it to get the full size.

This starts Row 4 Notice the repeating code for every row? It's important. A vertical line and a dash indicate the start of a new row. An exclamation point indicates the first column. You can put the entire row onto a single line, but it's easier to read if you break it up. Again, wiki usually ignores new paragraphs. Make sure to end the table correctly (vertical line and closed brace). Not doing so might still display the table, but nothing that comes afterwards.